I agree with the posters who have said to just keep them outside until they go…of course, that means you'll probably need to get wet too.
Ruby & Brando (4yo each) both learned early on that if it rains, they are still going out in it and getting walked in it (unless it is a major downpour - but even in that case, they go out when it has let up slightly). Brando minds the rain less than Ruby, but both realize they don't have a say in it...it just makes them do their business quicker. If I just let them out and didn't join them outside, I'm sure they would not go and would stay on the porch or under the porch. But because I too go out and get wet, they both get their business done (either on the walk or in the yard after the walk). I don't let them in the house until both have gone and in fact, won't even walk to the steps if one is holding out.
With Liyah (5mo), it is a little different. Not to rub it in...but Liyah LOVES the rain. She is no different about going out in the rain than she is if it is sunny. She sometimes stands there with her head up in the air just letting the rain hit her face :D:D. She explores the yard and even when she has done her business, she doesn't run right back to the steps to go in. It is pretty funny, because Ruby will run off the porch to check on her (and tell her to "hurry up") so that they can all go back in the house. Once I've seen Liyah do her business, if she doesn't want to come in, I leave her out in the rain. Because Liyah ends up getting soaked, both Ruby and Brando lick her dry when they are back inside.:D:D:D